From Wikipedia:
Most influenza outbreaks disproportionately kill the very young and the very old, with a higher survival rate for those in between,
but the Spanish flu pandemic resulted in a higher-than-expected mortality rate for young adults. Scientists offer several possible explanations for the high mortality rate
of the 1918 influenza pandemic. Some analyses have shown the virus to be particularly deadly because it triggers a cytokine storm, which ravages the stronger immune system of young adults. In contrast, a 2007 analysis of medical journals from the period of the pandemic found that the viral infection was no more aggressive than previous influenza strains. Instead, malnourishment, overcrowded medical camps and hospitals, and poor hygiene, all exacerbated by the recent war, promoted bacterial superinfection. This superinfection killed most of the victims, typically after a somewhat prolonged death bed.
The 1918 Spanish flu was the first of two pandemics caused by H1N1 influenza A virus; the second was the 2009 swine flu pandemic.
A 2020 study found that US cities that implemented early and extensive non-medical measures (quarantine, etc.) suffered no additional adverse economic effects
due to implementing those measures, when compared with cities that implemented measures late or not at all.
While systems for alerting public health authorities of infectious spread did exist in 1918, they did not generally include influenza, leading to a delayed response.
Nevertheless, actions were taken. Maritime quarantines were declared on islands such as Iceland, Australia, and American Samoa, saving many lives.
Social distancing measures were introduced, for example closing schools, theatres, and places of worship, limiting public transportation, and banning mass gatherings.
Wearing face masks became common in some places, such as Japan, though there were debates over their efficacy. There was also some resistance to their use, as exemplified by
the Anti-Mask League of San Francisco. Vaccines were also developed, but as these were based on bacteria and not the actual virus, they could only help with secondary infections.
The actual enforcement of various restrictions varied. To a large extent, the New York City health commissioner ordered businesses to open and close on staggered shifts to avoid overcrowding on the subways.
A later study found that measures such as banning mass gatherings and requiring the wearing of face masks could cut the death rate up to 50 percent, but this was dependent on them being imposed early in the outbreak and not being lifted prematurely.
